The present invention relates to an antenna element suitable for radio systems such as portable radio communications apparatuses using microwaves, quasi microwaves or millimeteric waves, and particularly to an antenna element that is to be mounted in microwave radio communications apparatuses for an intelligent transport system (ITS), etc. such as vehicle information and communications systems (VICS) and electronic toll collection systems (ETC), and further to a radio communications apparatus comprising such an antenna element.
According to recent demand of miniaturization and cost reduction of microwave radio communications apparatuses, there is strong demand to reduce the size of antennas mounted in microwave radio communications apparatuses. For instance, used in cellular phones are generally monopole antennas that are retractable in bodies of cellular phones. From the aspect of improvement in portability, further miniaturization and reduction in weight of antennas and the mounting of antennas in bodies of phones are desired.
Widely used as built-in antennas are conventionally inverted F-type antennas and micro-strip antennas that are constituted by monopole antennas bent in parallel with a ground for miniaturization and reduction in thickness. However, because an antenna of this type utilizes a phone body as a ground, the dimension of a phone body affects the radiation directionality of an antenna, and part of electric current induced in a phone body by the radiation of electromagnetic waves from the antenna flows into a hand of a person holding the phone. Also, because sufficient bandwidth and gain are not obtained, the overall size of the antenna should be large to obtain bandwidth and gain almost comparable to those of the monopole antenna, whereby it cannot easily be installed in small radio communications apparatuses such as recent cellular phones.
Thus, pole antennas are disadvantageous in inconvenience and restriction in the freedom of design. Therefore, a coaxial resonant slot antenna having a structure in which a strip conductor is disposed in an internal space of a flat box-shaped conductor cubic in an insulated manner so that it is operable in a transverse electromagnetic mode (TEM) was proposed (U.S. Pat. No. 5,914,693). The structure of this slot antenna is shown in FIGS. 50 (a) and (b). This slot antenna is constituted by bonding an insulating substrate 501a having a slot 503 formed by the pattern etching of a conductor layer 502 to an insulating substrate 501b having a strip conductor layer 504 formed by the etching of a conductor layer.
When transmission is carried out by this coaxial resonant slot antenna, a high-frequency signal supplied from a feeder flows through a strip conductor layer 504 to the slot 503, from which it is radiated to the sky by a resonance phenomenon of the slot 503. Also, in the case of reception, an electromagnetic wave (received signal) introduced into the conductor cubic through the slot 503 progresses through the strip conductor layer 504 in a direction opposite to the above direction and picked up by a feeder as a high-frequency signal.
However, because this coaxial resonant slot antenna has a structure in which an insulating substrate 501a having a slot 503 is bonded to an insulating substrate 501b having a strip conductor layer 504, an electromagnetic coupling coefficient is susceptible to variation due to relative displacement between the slot 503 and the strip conductor layer 504 that is likely to occur in their bonding step, resulting in large variation in a resonance frequency and a voltage-standing wave ratio (VSWR) representing the condition of impedance matching. To reduce variation in VSWR, the insulating substrates 501a, 501b, the slot 503 and the strip conductor layer 504 should be formed at high precision, and the insulating substrates 501a and 501b should also be bonded precisely, resulting in complicated production processes.
In the case of mounting a slot antenna, an apparent impedance of the antenna varies by a floating capacitance between a ground pattern in contact with the slot antenna and a body of a microwave radio communications apparatus. Accordingly, impedance matching should be achieved between the slot antenna and the feeder system. However, because the above coaxial resonant slot antenna has a structure in which the strip conductor layer is disposed in a conductor cubic, the impedance matching cannot easily be achieved. In addition, because the coaxial resonant slot antenna should be designed in a manner matching with the shapes of the board and body of the microwave radio communications apparatus, there arises a problem of extremely increased production cost in cellular phones, etc. whose specifications are frequently modified.
In addition to the above coaxial resonant slot antenna, there is a rectangular hollow slot antenna having a shape shown in FIG. 51 (see xe2x80x9cAntenna Engineering Handbook,xe2x80x9d page 89). This rectangular hollow slot antenna comprises a slot 3 on an upper surface of a first flat conductor 2, and high-frequency electric power terminals OSCs at both ends of the slot 3, and OSCs serve to receive a signal and emit radio waves.
The specifications required for the built-in antennas as described above depend on systems in which they are used. For instance, in US-PCS (the United States) or K-PCS (Korea) utilizing a cellular phone system of 1.9 GHz band (2 GHz band in the global standard CDMA system), frequency bandwidths shown in Table 1 below are necessary.
However, the rectangular hollow slot antenna as shown in FIG. 51 does not meet the above specifications of antennas. Also, because a wider bandwidth is more effective to prevent the deterioration of performance due to the variation of use environment in a small antenna mounted in a cellular phone, it is important to expand the bandwidth of the antenna. The principle for expanding bandwidth is as follows. At a bandwidth Bw, the following equation is met.
Bwxe2x88x9dxe2x88x921/Qxe2x80x83xe2x80x83(1).
Thus, the smaller the Q, the more the bandwidth Bw expands. Also, at a radiation efficiency xcex7, the following equation is met.
xcex7=1/(1+Qr/Qi)xe2x80x83xe2x80x83(2).
wherein Qi=Qc+Qd, Qc and Qd are the values of Q by conductor loss and dielectric loss, respectively, and Qr is the value of Q by radiation. Therefore, when Qr is small, the radiation efficiency xcex7 is large.
Thus, the element should have a small Q to expand the bandwidth, and Qr should be small to increase the radiation efficiency xcex7. For instance, in the case of an antenna for a cellular phone, the bandwidth of 20 MHz is needed.
xe2x80x83Qxe2x88x9dxcfx89Cxe2x80x83xe2x80x83(3),
C=axc2x7xcex5rxe2x80x83xe2x80x83(4),
wherein xcfx89 is an angular frequency, C is capacitance, a is a constant determined by the antenna shape, and xcex5 is a dielectric constant.
The following relation is satisfied from the equations (3) and (4).
Qxe2x88x9dxcfx89xc2x7axc2x7xcex5rxe2x80x83xe2x80x83(5).
It is known from the equation (5) that a material having a small dielectric constant should be used to keep the Q low.
Also, there is a relation between Qr and the thickness (height) of an antenna, which is
Qrxe2x88x9d1/txe2x80x83xe2x80x83(6),
wherein t is a thickness (height) of an antenna. Thus, the antenna should be made thick to decrease Qr.
Further, the slot antenna having a shape shown in FIG. 51 is disadvantageous in that power of radio waves emitted from the antenna (radiation gain) is small. Thus, in the conventional slot antenna, a material having a small dielectric constant such as a glass-filled epoxy resin, Teflon, etc. is used to decrease the Q, thereby increasing the bandwidth. Here, assuming that the slot shown in FIG. 51 has a length L, the following relations are satisfied.
L=xcex/2xe2x80x83xe2x80x83(7),
xcex=xcex0/xcex5xe2x80x83xe2x80x83(8),
xcex5=(1+xcex5r)/2xe2x80x83xe2x80x83(9),
wherein xcex0 is a wavelength in vacuum, xcex is a wavelength compressed by a dielectric body, xcex5 is an effective dielectric constant, and xcex5r is a dielectric constant.
Because a material having a small dielectric constant provides a small wavelength compression ratio, a signal resonating in the antenna has a short wavelength, thereby failing to reduce the length L of the antenna slot. Also, when a material having a large dielectric constant is used, the antenna should be made thick to decrease the Q, thereby expanding the bandwidth. However, when a high slot antenna is mounted inside a cellular phone, the cellular phone inevitably becomes large, resulting in the reduced freedom of design. In addition, a small radiation gain makes it impossible to send radio waves far enough, causing increase in transmission error. Accordingly, an antenna having a large radiation gain is necessary.
Accordingly, an object of the present invention is to provide a small, thin antenna element having a low Q and large frequency bandwidth and radiation gain, particularly a slot antenna free from large variations of resonance frequency and VSWR and easily achieving impedance matching with a feeder, which is produced with a small number of steps.
As a result of intense research in view of the above object, the inventors have found that a slot antenna-type antenna element constituted by a radiation conductor layer formed on an upper surface of an insulating substrate, a slot provided on an upper surface and/or a side surface of the insulating substrate, and a strip conductor layer formed inside the slot such that it is electrically insulated from the radiation conductor layer is small and thin, providing low Q and large frequency bandwidth and radiation gain, and that at least one slit gap provided in a radiation conductor layer for dividing the radiation conductor layer in an electric current direction serves to further reduce Q of the antenna element, thereby further increasing the frequency bandwidth and the radiation gain. The present invention has been completed based on these findings.
Thus, the antenna element of the present invention comprises an insulating substrate; a first conductor layer formed continuously on an upper surface, a bottom surface and at least one side surface of the insulating substrate; a slot consisting of a portion in which a conductor layer is not formed on an upper surface and/or a side surface of the insulating substrate; and a second strip conductor layer formed in the slot or in an insulated extension connected to the slot thereby being electrically insulated from the first conductor layer, the second conductor layer being electrically connected to a feeder. With such a constitution, an electromagnetic coupling coefficient between the second strip conductor layer and the slot is easily controlled.
The second strip conductor layer preferably has at a tip end an extension that is trimmed to control the length of the second strip conductor layer. With the length of the strip conductor layer controlled by trimming the extension by laser or other means, impedance matching with the feeder can easily be achieved.
The slot is preferably provided on at least one side surface of the insulating substrate. The second conductor layer preferably exists in the slot at a position opposite to the side surface of the insulating substrate on which the first conductor layer is formed.
The first conductor layer formed on an upper surface of the insulating substrate is preferably divided by at least one slit gap extending substantially in perpendicular to the electric current direction at a position separated from the slot. A radiation region of the first conductor layer is preferably divided into three parts by a plurality of slit gaps. Also, the slit gap is preferably in parallel with the slot.
A ratio Sb/Sa of an area Sb of the slit gap to an area Sa of the radiation region of the first conductor layer is preferably 0.05 or more. Also, a ratio c/a of a distance c from the second conductor layer to the slit gap to a distance xcex1 from the second conductor layer to the side surface of the insulating substrate, on which the first conductor layer is formed, is preferably 0.1 or more.
The insulating substrate of the above antenna element is preferably made of a ceramic based on alumina or calcium zirconate.